Such ball screw mechanisms have been known for many years. They comprise a recirculating unit for returning, by one or more threads each, balls that circulate in grooves between a worm spindle and a nut. External recirculator tubes are customary with coarse thread or multithread arrangements. A recirculator without a solid base as described in DE-U-8 108 413 comprises a cage-like guide frame composed of webs and bridges. A design disclosed in DE-C-2 355 844 features a recirculator extending through the nut, with radial projections matching the worm grooves.
There are problems as to fitting accuracy and constancy of registration with such ball screw mechanisms. They should be free of steps and gaps in order to prevent hindrances to the ball circulation. However, that can hardly be achieved owing to inevitable tolerances of production and assembly; moreover, the recirculators must be adapted to be exchanged by a replacement unit after wear and tear. Also, exact fit in respect of the angular position relative to the worm is not automatically warranted so that expensive measures to correct the adjustment may become necessary.